linux-kernel - Re: [PATCH] perf/core: introduce context per CPU event list

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Date:   Thu, 10 Nov 2016 11:05:17 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     kan.liang@...el.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, acme@...nel.org, tglx@...utronix.de,
        alexander.shishkin@...ux.intel.com, vince@...ter.net,
        eranian@...gle.com, andi@...stfloor.org
Subject: Re: [PATCH] perf/core: introduce context per CPU event list

Hi,

On Thu, Nov 10, 2016 at 09:33:55AM +0100, Peter Zijlstra wrote:
> Yes this is a problem, but no this cannot be done. We can't have per-cpu
> storage per task. That rapidly explodes.
> 
> Mark is looking at replacing this stuff with an rb-tree for big-little,
> that would also allow improving this I think.

Unfortunately I've not had the chance to look at that since returning
from Plumbers. Also, I was leaning towards the alternative approach we
discussed, with a perf_event_task_contexts container, as that also
solved some other issues with the way we used perf_event_context::pmu in
big.LITTLE systems.

Looking at the way perf_iterate_ctx is used, it seems that we're just
trying to iterate over the active events for a context (i.e. those
programmed into the HW at this point in time). Though I'm missing some
subtlety, since we check event->state < PERF_EVENT_STATE_INACTIVE.

We have a similar issue with perf_event_task_tick() needing to know the
relevant contexts, and for that we have the active_ctx_list. Can't we do
something similar and add an active_events_list to perf_event_context?

Thanks,
Mark.